The line of sight of a first device termed the transmitter in a frame tied to a second device termed the receiver is by definition the direction in said frame of the receiver device of a vector connecting the center of said frame of the receiver device to the center of a frame tied to the transmitter device.
The knowledge of the line of sight between a first vehicle carrying the transmitter device and a second vehicle carrying the receiver device is used in numerous applications, in particular in aerospace systems.
For example, the knowledge of the line of sight between two vehicles, associated with knowledge of a distance between said two vehicles, makes it possible to determine the relative position of the first vehicle with respect to the second vehicle. The knowledge of the relative position is particularly useful in the case of space vehicles performing an autonomous rendezvous mission in orbit (the vehicles approaching one another up to contact), or a formation flying mission (the vehicles having to stabilize themselves precisely in a fixed position with respect to one another). In these two examples, the distance between the two vehicles generally lies between a few meters and a few kilometers.
Another application of the knowledge of the line of sight is attitude determination, which can be obtained on the basis of the knowledge of the lines of sight between the receiver device whose attitude one wishes to determine, and at least two transmitter devices whose relative positions are known (such as GPS satellites) and such that the corresponding lines of sight are substantially different.
In practice, in these applications, the uncertainty in the line of sight between the transmitter and receiver devices must be as small as possible, preferably between a few tenths of degrees and a few degrees. Moreover, the determination of the line of sight must be carried out in the simplest possible manner without introducing overly significant design constraints on the transmitter and receiver devices.
Known methods for determining line of sight are based on angular phase measurements of radioelectric signals transmitted by a transmission antenna of the transmitter device and received by a base of reception antennas of the receiver device, that is to say a pair of reception antennas R1 and R2, separated by a distance L12 which generally lies between a few tens of centimeters and a few meters. The radioelectric signals comprise at least one periodic component of fixed and known wavelength λ1.
The known methods based on angular phase measurements use a measurement Δ{tilde over (φ)}112 of a difference of the angular phases of the radioelectric signals received on the two reception antennas, said angular phases being measured modulo 2π. A measurement Δ{tilde over (φ)}112 of linear phase difference, dimensionally equivalent to a distance, is obtained on the basis of the measurement Δ{tilde over (φ)}112 of angular phase difference by calculating:
      Δ    ⁢                  ϕ        ~            1      12        =                    Δ        ⁢                              φ            ~                    1          12                ⁢                  λ          1                            2        ⁢        π              .  
In the case where the distance between the transmitter and receiver devices is large compared with the distance L12, typically by a factor of 10 or more, the linear phase difference depends essentially on a difference in path length d12 of the radioelectric signals received on the two reception antennas R1 and R2. The knowledge of said difference in path length makes it possible to determine an angle of sight which is the angle between the straight line joining the radioelectric centers of the two reception antennas and the straight line joining the radioelectric center of one of the two reception antennas to the radioelectric center of the transmission antenna.
The use of at least three nonaligned reception antennas makes it possible to determine at least two angles of sight which completely define the line of sight.
An essential difficulty with these methods is to do with the fact that the angular phases are measured modulo 2π, that is to say they are known only to within an integer number of times 2π, the integer number being a priori unknown. To determine the difference in path length d12, the ambiguity in the measurement Δ{tilde over (φ)}112 of linear phase difference must be resolved by determining the corresponding integer.
To resolve the ambiguity in the measurement Δ{tilde over (φ)}112 of linear phase difference, it is known from a method, termed “null space” in the literature, to carry out a systematic exploration of all the combinations of integers and to determine the most likely combination. This method exhibits the drawback of being complex and of requiring high calculation power.
It is also known from a method, termed “motion based”, to impress a known rotation motion on the vehicle carrying the receiver device, and to use this motion to resolve the ambiguity. In this case, the implementation of the method imposes significant constraints on the operations of the system, and introduces a lag in performing the rotation motion during which the line of sight may change.
A method, termed “pseudo-distance” or “pseudo-range”, determines the difference in path length directly on the basis of measurements of the propagation times between the transmission antenna of the transmitter device and the reception antennas of the receiver device.
By directly determining the difference in path length without using the measurement Δ{tilde over (φ)}112 of linear phase difference, there is no ambiguity to be resolved. However, the estimate of the difference in path length thus obtained is very noisy since in practice it is difficult to obtain propagation time measurements with good accuracy, in particular in the presence of multipaths between the transmitter device and the receiver device.